1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method of suppressing interferences between cells.
2. Description of the Related Art
In a mobile communication system, interferences between peripheral cells are a serious problem.
In order to solve this problem, recent mobile communication systems that employ a CDMA (Code Division Multiple Access) scheme code-spread transmission signals using codes that differ from cell to cell or from user to user, and despread the spread transmission signals using a spread gain at the reception side to relatively suppress interference signals However, in a case where codes between users are not perfectly orthogonal to one another, such as on uplink lines in a WCDMA (Widened CDMA) scheme, signals from other users can cause interferences, resulting in a reduction in the number of users that can be accommodated in a cell, as compared with a case where codes between users are orthogonal to one another. As a result, the sum of data speeds of all users in a cell, i.e., the cell capacity, decreases.
In a FDMA (Frequency Division Multiple Access) mobile communication system prior to the advent of a CDMA mobile communication system, frequencies which differ between neighboring cells are allocated to user equipments, to avoid interferences. However, this in turn causes unavailable frequencies for each cell, thus decreasing the efficiency of use of frequencies.
In the next mobile communication system, it is considered that, in order to carry out orthogonalization among users, the same frequency is used even among neighboring cells to apply the FDMA scheme for uplink signals and to avoid a decrease in the efficiency of use of frequencies. Although it is expected that interferences among cells can be avoided to some extent by using a retransmission scheme, such as HARQ (Hybrid Automatic Repeat reQuest), further improvements are desired.
Documents that are related to the technique described above include: “Spectrum spread communication and its application” written by G. Marubayashi, M. Nakagawa, R. Kohno, published Electronics, Information, and Communication Society, May 10, 1998, P. 188˜199, and “Foundations of mobile communications” editorially supervised by Y. Okumura, M. Shinshi supervised, published by Electronics, Information, and Communication Society, Oct. 1, 1986, P. 188˜195